1. Field of the Invention
The invention relates generally to wood burning appliances but more particularly to an adjustable control that provides variable air intake resulting in efficient and clean burning.
2. Background of the Invention
Wood burning appliances must conform to Environmental Protection Agency (EPA) standards for emission control. Appliances are tested based on a range of burn rates from the minimum to the maximum. The test starts by preheating the appliance in a predetermined manner, and then follows a test procedure as specified in the EPA standards.
EPA imposes the following restrictions and requirements:
Restriction: The operator cannot make adjustments of the combustion air after the first five minutes of the test.
Requirements: The appliance must be able to burn less than 1 kg of wood per hour while producing less than 7.5 grams of pollutants per hour.
The following problems arise:
For a low burn rate (less than 1 kg per hour), the combustion air must be set to a minimum. In this case, the time allowed to start the fire (5 minutes) is not sufficient. Therefore, the wood does not burn properly producing excess pollutants.
In order to account for that, it is necessary to have a variable air intake or preferably a second air intake that is used for starting a fire quickly and which can be shut down when the fire is considered to be in a condition for continued burning with sufficient combustible material and air intake which provides for a burn in accordance with EPA standards.
There are a number of slow combustion stoves disclosed in the prior art, they generally make use of a thermostat to actuate changes in the combustion process such as bringing in more fuel or changing the air-fuel ratio. Such examples can be found in U.S. Pat. No. 4,052,136 wherein a thermostat is arranged to be responsive to the temperature of flue gases from an oil burning heater and actuates switches when the temperature of the flue gases reaches either a predetermined high value or a predetermined low value.
U.S. Pat. No. 4,409,956 also describes a thermostat for a stove wherein the thermal control unit is contained in a housing which has an aperture formed therein and includes a thermal control unit for detecting changes in the temperature of the stove. A flap is secured to the housing and is rotatable across the aperture so as to cover the aperture to any desired degree.
U.S. Pat. No. 4,530,346 shows a mechanical thermostat for modulating the temperature of a solid fuel-burning stove, including a casing, a thermostat coil mounted in the casing to sense the stove wall plate temperature; a downwardly-spaced air admission mechanism. The air admission mechanism includes a pair of identical openable and closable orifices by way of a variable closure mechanism.
The problem with thermostat is that they are merely switches actuating a generally electromechanical process which will effect the required change. This involves a plurality of components, any one of which can be prone to failure.